The Zend_XmlRpc_Value class also appears to have done away with the struct type. So associative arrays are converted to strings too. Note the _phpVarToNativeXmlRpc method and how it is missing a case for 'struct' in the switch statement

switch (self::getXmlRpcTypeByValue($value))

line 318

There is an array type, but no struct type even though the array case very explicitly shows Struct code.